Other numbers are as follows and not declined, except for those that are actually nouns:
five
panca
nineteen
nítján
six
sekes
twenty
tuttugu
seven
seter
twenty-one
tuttugu og einn
eight
siveno
thirty
þrjátíu
nine
mineth
forty
fjörutíu
ten
tiz
fifty
fimmtíu
eleven
ellefu
sixty
sextíu
twelve
tólf
seventy
sjötíu
thirteen
þrettán
eighty
áttatíu
fourteen
fjórtán
ninety
níutíu
fifteen
fimmtán
(one) hundred
(irke) sto
sixteen
sextán
(one) thousand
(irke) tisic
seventeen
sautján
(one) million
(irke) miliun
eighteen
átján
zero
nul
The word sto is actually a neuter noun, tisic can be either feminine or neuter and the higher multiples of a thousand are either masculine or feminine, according to the ending (e.g. milljón is feminine, milljarður is masculine and so on). Nul is neuter.
Adjectives must agree with the gender, number and grammatical case of the nouns they describe. For example, the word íslenskur (Icelandic) agrees as follows:
Hokian (strong)
case
Masculine
Feminine
Neuter
singular
nom.
hoki
íslensk
íslenskt
acc.
íslenskan
íslenska
dat.
íslenskum
íslenskri
íslensku
gen.
íslensks
íslenskrar
íslensks
plural
nom.
íslenskir
íslenskar
íslensk
acc.
íslenska
dat.
íslenskum
gen.
íslenskra
In strong declension, for example:
Ég bý með íslenskri konu—I live with an Icelandic woman